Datasheet

10
20 30
40
2500
2000
1500
1000
500
0
f -SwitchingFrequency-kHz
s
V -InputVoltage-V
I
Skip
Shift
V =3.3V
O
( )
( )
L OUTSC
SW shift
ON IN L
(I Rdc V Vd)
div
t V I x Rhs Vd
æ ö
´ + +
= ´
ç ÷
ç ÷
- +
è ø
f
f
( )
( )
( )
L OUT
SW max skip
ON IN L
I Rdc V Vd
1
t V I Rhs Vd
æ ö
´ + +
æ ö
= ´
ç ÷
ç ÷
ç ÷
- ´ +
è ø
è ø
f
TPS54240
www.ti.com SLVSAA6A –APRIL 2010–REVISED DECEMBER 2012
DETAILED DESCRIPTION (continued)
Selecting the Switching Frequency
The switching frequency that is selected should be the lower value of the two equations, Equation 12 and
Equation 13. Equation 12 is the maximum switching frequency limitation set by the minimum controllable on time.
Setting the switching frequency above this value will cause the regulator to skip switching pulses.
Equation 13 is the maximum switching frequency limit set by the frequency shift protection. To have adequate
output short circuit protection at high input voltages, the switching frequency should be set to be less than the
fsw(maxshift) frequency. In Equation 13, to calculate the maximum switching frequency one must take into
account that the output voltage decreases from the nominal voltage to 0 volts, the fdiv integer increases from 1 to
8 corresponding to the frequency shift.
In Figure 40, the solid line illustrates a typical safe operating area regarding frequency shift and assumes the
output voltage is zero volts, and the resistance of the inductor is 0.130Ω, FET on resistance of 0.2Ω and the
diode voltage drop is 0.5V. The dashed line is the maximum switching frequency to avoid pulse skipping. Enter
these equations in a spreadsheet or other software or use the SwitcherPro design software to determine the
switching frequency.
(12)
(13)
I
L
inductor current
Rdc inductor resistance
V
IN
maximum input voltage
V
OUT
output voltage
V
OUTSC
output voltage during short
Vd diode voltage drop
R
DS(on)
switch on resistance
t
ON
controllable on time
ƒ
DIV
frequency divide equals (1, 2, 4, or 8)
spacer
spacer
Figure 40. Maximum Switching Frequency vs. Input Voltage
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